1. Field of the Invention
The present invention relates to a methodology for enhancing bar codes formed in an impact printer by increasing the density of selected print bars or lines, and increasing the efficiency of the printing operation by extending the useful life of the print ribbons. The enhancing methodology may occur with the insertion of a new ribbon into the printer, or may be commenced at or near the print ribbon's "end of life" stage. More particularly, the present invention relates to a methodology of printing bar codes wherein the vertical printing element (PEL) density is selectively increased and/or double strike or overstrike techniques are utilized to form readable bar codes while extending ribbon life.
2. Description of the Prior Art
In recent years, it has become increasingly important to print special symbols, such as bar codes, in an impact printer in a consistent, uniform, recognizable and economical manner. This is particularly true for use of today's ubiquitous bar codes which depict various codes through the use of combinations of parallel narrow and thick vertical lines separated by spaces between the lines. Accurate recognition of bar codes by scanners and similar reading devices starts with the printing device that creates the bar codes and it is, therefore, important that printers produce readable bar codes. However, as important as the creation of readable bar codes may be, the costs of printing and down time for ribbon changes must be considered against the problems created by unsatisfactory bar codes.
Typical bar code printing may be accomplished in a dot matrix impact line printer, in a thermal printer or in other non-impact printers. This invention is concerned with the use of an impact line matrix printer that employs a fabric ribbon to print bar codes. Many printers of this type utilize a ribbon drive having opposed spools located adjacent the ends of a printing zone. The print zone usually includes a platen to support print receiving medium (conventionally a paper product) on which printing is to occur and a shuttle which carries a print head comprising a plurality of individually actuable or energizeable print hammers disposed to strike the ribbon and press the ribbon portion adjacent the head and media against the print receiving medium. Depending upon the printer design, the shuttle is driven from one edge of the media to the other edge thereof. Motion to effect relative perpendicular medium displacement, relative to the print head and ribbon (in effect a line feed), occurs in a conventional manner, e.g. a medium drive motor.
During printing operations, the ribbon is rotatably driven from one spool to the other through the print zone. Upon sensing that the end of the ribbon has been reached, the ribbon direction is reversed and the ribbon is fed through the print zone in the opposite direction.
Although there are several types of ribbons that can be utilized in a line matrix printer, the present invention is concerned with the use of fabric ribbons or ribbons made of a material in which ink can freely migrate into previously impacted areas of the ribbon from adjacent areas. When overall ink depletion in such ribbons reaches a level where migration is no longer possible or insufficient ink remains to replenish what has been used, the ribbon is said to have reached end of life. Other factors, such as mechanical wear or the printer's operating environmental conditions, for example, can also affect ribbon life.
For purposes of printing bar codes, however, end of ribbon life is a function of the suitability of the bar codes being printed to be properly recognized by a bar code reader, which may change for the worse long before ribbon ink is depleted or mechanical wear affects the ribbon's bar code suitability. The amount of ink left in a ribbon or the expectation that the ink left will flow to ink depleted areas of the ribbon are relatively unimportant compared to the suitability of the bar codes being produced by a particular ribbon. Thus, ribbon life needs to be determined in a different manner when bar codes are to be printed.
There are several methods of determining ribbon condition with respect to printing of bar codes. In one approach, a counter is used to add up all print impacts and to trigger a "change ribbon" signal when the count reaches a predetermined number. See U.S. Pat. No. 5,092,695 to Silverman et al, "Printer Having Ribbon Wear Indicator" for some examples of this approach. An alternative scheme is described in Japanese Patent No. 61-32772 to Ueno, "Bar-Code Printing Apparatus" wherein a scanner reads and compares each bar code after it is printed to a standard bar code and flashes a ribbon replacement indicator when the two codes are sufficiently different. In yet another approach, the difference in contrast between the bar code lines and their interlineated spaces, is measured and the ribbon changed whenever print contrast falls below a reading of 2.0 ANSI grade and/or the print contrast signal falls below 0.70 PCS, levels at which bar code readings become questionable and error plagued when scanned. (The ANSI standard specifications for Bar Code are set forth in "Bar Code Print Quality Guide", ANSI X-3.182-1990(R1995)).
This low contrast problem is actually more pronounced with the single dot width narrow bars or lines of a bar code than they are with the wider multiple dot bars which have horizontal overlaps of their constituent adjacent dots. It has been found, when a low contrast condition of this type occurs, that most ribbons are still in reasonable and usable condition and have not incurred any permanent damage or ink depletion that would otherwise prevent their continued use for other than bar code purposes.